Referring to FIG. 1, which shows a conventional muting circuit design for audio amplifier. A pair of CMOS switchs M1 and M2 is added in the audio path of the audio amplifier, in which M1 is an NMOS, while M2 is a PMOS. The audio signal Vin is inputted to the drains of the CMOS switchs M1 and M2, then passes through the sources of the CMOS switchs M1 and M2 to be inputted to the amplifier OP4 for generating an output signal Vout. A muting signal M0 passes through two inverters INV5 and INV6 for controlling the gates of M1 and M2 respectively, and controlling a bias switch M3, as shown in the figure.
When the muting signal M0 is “1” (high voltage) (means performing the muting function), the CMOS switchs M1 and M2 will be opened, M3 will be conducting, the audio signal Vin will be blocked, and a constant bias Vreference will be inputted to the amplifier OP4, thus forming the muting effect. The equivalent circuit thereof is shown in FIG. 2.
When the muting signal M0 is “0” (low voltage) (means no muting), the CMOS switchs M1 and M2 will be conducting, M3 will be opened, the audio signal Vin passes through the CMOS switchs M1 and M2 to be inputted to the amplifier OP4. The equivalent circuit thereof is shown in FIG. 3, in which the CMOS switchs M1 and M2 become a resistor Ron, M3 is opened without any effect, the input resistance of the amplifier OP4 is Ri, therefore the input signal Vi of the amplifier OP4 equals to Vin×Ri/(Ron+Ri).
Referring to FIG. 4, which shows the resistance characteristics of the CMOS switchs M1 and M2. RM1 represents the characteristic curve of the resistance of switch M1 according to the variation of the voltage. RM2 represents the characteristic curve of the resistance of switch M2 according to the variation of the voltage. Ron represents the characteristic curve of the combined resistance of switchs M1 and M2 according to the variation of the voltage when the input signal is small. Since RM1, RM2 and Ron vary according to the voltage, Vi varies according to the variation of Ron, and causes input distortion to the amplifier OP4. Therefore this muting circuit is not a good design.